Thermal inkjet printers operate by rapidly heating a small volume of ink and causing the ink to vaporize, thereby ejecting a droplet of ink through an orifice to strike a recording medium, such as a sheet of paper. When a number of orifices are arranged in a pattern, the properly sequenced ejection of ink from each orifice causes characters or other images to be printed upon the paper as the printhead is moved relative to the paper.
In these printers, print quality depends upon the physical characteristics of the orifices, or nozzles, in the printhead. For example, the geometry of the nozzles affects the size, shape, trajectory, and speed of the ink drop ejected.
FIG. 1 is a cross-section of a desirable type of thermal inkjet printhead 8. Printhead 8 includes a nozzle member 10, having a tapered nozzle 12. Affixed to a back surface of nozzle member 10 is a barrier layer 14, which channels liquid ink into a vaporization chamber 16. Liquid ink within vaporization chamber 16 is vaporized by the energization of a thin film resistor 18 formed on the surface of a semiconductor substrate 20, which causes a droplet of ink 22 to be ejected from nozzle 12.
Preferably, nozzle member 10 is formed of a polymer material, and nozzle 12 is formed in nozzle member 10 using laser ablation. Nozzle member 10 can also be formed of a photoresist material, where nozzle 12 is formed using photolithographic techniques or other techniques.
Tapered nozzles have many advantages over straight-bore nozzles. A tapered nozzle increases the velocity of an ejected ink droplet. Also, the wider bottom opening in the nozzle member 10 allows for a greater alignment tolerance between the nozzle member 10 and the thin film resistor 18, without affecting the quality of print. Additionally, a finer ink droplet is ejected, enabling more precise printing. Other advantages exist.
If nozzle 12 is to be formed using a laser, a tapered nozzle 12 may be formed by changing the angle of nozzle member 10 with respect to a masked laser beam during the orifice forming process. Another technique may be to use two or more masks for forming a single array of nozzles 12 where each mask would have a pattern corresponding to a different nozzle diameter. Still another technique is to defocus the laser beam during the orifice forming process. European Patent Application 367,541 by Canon describes such a defocusing technique and other techniques for forming tapered nozzles using a laser. U.S. Pat. No. 4,940,881 to Sheets describes still another technique for forming tapered nozzles with a laser by rotating and tilting an optical element between the laser and the nozzle plate. These various techniques are considered time consuming, complicated, and subject to error.
FIG. 2 illustrates a conventional mask portion 24 having an opening 26 corresponding to where a nozzle is to be formed in a nozzle member. The opaque portion 28 of the mask is shown as being shaded. These conventional masks have been used in the past, in conjunction with various laser exposure techniques, for forming straight and single-angled tapered nozzles by controlling the fluence (mj/cm.sup.2) of laser radiation at the target substrate.
U.S. Pat. No. 4,558,333 to Sugitani et al. describes a photolithographic process using a single mask to form tapered nozzles in a photoresist. The tapering is due to the opaque portions of the mask causing frustum shaped shadows through the photoresist layer corresponding to where nozzles are to be formed. After developing and etching the photoresist, the resulting nozzles have a frustum shape. The mask used is similar to that of FIG. 2 but where the opaque portion 28 and clear portion 26 are reversed.
This relatively simple method for forming tapered nozzles in photoresist nozzle members, using a single conventional mask, cannot be used for forming tapered nozzles in a polymer nozzle member using laser ablation.
Accordingly, what is needed is a highly reliable method and apparatus for forming tapered nozzles in a polymer nozzle member using laser ablation.